Vibration and drag reduction system for fluid-submersed hulls

ABSTRACT

A system for reducing hydrodynamic drag and vortex-induced-vibration (&#34;VIV&#34;) in a bluff hull. In a bluff hull that is designed to be at least partially submerged in a fluid, such as water, the hull has an internal area for holding or transferring fluid. When the hull is beset by a current present in the water, the surface of the bluff hull has an up-current side and a down-current side. The hull surface has at least one opening, preferably a slot-nozzle, for blowing the fluid from the internal fluid area of the bluff hull out of the hull surface and into the surrounding water at a velocity greater than the current velocity, so as to reduce flow separation of the current on the down-current side of the hull surface. Also, preferably, the fluid is blown out of the opening at an angle substantially tangential to the hull surface at the location of the opening and substantially in the direction of the current.

TECHNICAL FIELD

This invention relates to a system for reducing vibrations and drag influid-submersed hulls. More particularly, the invention relates to aboundary layer control system that reduces the hydrodynamic drag andshed-vortex induced vibrations on bluff hulls at least partiallysubmersed in water, such as SPARs, marine risers that connect a floatingdrilling vessel to the ocean floor, and semi-submersible ocean drillingvessels.

BACKGROUND

FIG. 1A is a side-view of a SPAR 100. The SPAR 100 is a largeun-propelled vessel of generally circular-cylindrical form that isoriented in the sea 102 with its long as 104 vertical. When ballasted,the SPAR 100 exhibits a very deep draft in comparison with its diameter106 and/or its freeboard 108, thus providing a very stable platform oflarge volume for offshore petroleum production and storage. SPAR vesselscan be positioned and restrained by an elaborate system of moorings,such as moorings 110, which may be anchored to the ocean floor 111.

When beset by ocean currents 112, a SPAR 100 will exhibit substantialdrag forces and large scale, long period, shed-vortex induced vibrations("VIV"). As shown in FIG. 1B, which is a top-view of SPAR 100, VIV isinduced when currents 112 travel around the hull of the SPAR 100,forming a down-current pressure gradient 114 and assymetrical circulareddies 116. The assymetrical nature of these eddies 116 causes the SPAR100 to oscillate orthogonally relative to the currents 112 Theseoscillations are known as VIV.

A system that has been used to attenuate such VIV is the addition oflarge scale helical "strakes" 118 to the exterior of the hull of theSPAR 100. While the addition of the strakes 118 may reduce VIV, thestrakes 118 actually increase drag and cause the mooring systems 110 tobe overwhelmed in the face of currents. This forces the development anduse of means other than the strakes 118 to reduce drag and relieve thestress on mooring systems.

FIGS. 2A and 2B show a marine riser 200. Marine risers 200 are used toconnect a floating drilling vessel 202 to the ocean floor 204 and toprovide a conduit for a drill string and drilling fluids. Like SPAR 100,when beset by ocean currents 206, marine riser 200 will exhibitsubstantial hydrodynamic drag forces and VIV. Such forces and motionsinduce mechanical stresses in, and deflections of, the marine riser 200and its connection 210 to the drilling vessel 202 and connection 212 tothe ocean floor 204, which ultimately may result in failure orinterference with drilling operations.

Drag and VIV have been reduced by the application of fairings 214 to themarine riser 200. The fairings 214 are enabled passively to rotate aboutthe riser 200 in order to align with the direction of the current 206 tominimize drag. While some drag and VIV reduction is thereby obtained,the procedure for applying and removing fairing segments from riserjoints while they are being run and retrieved is lengthy. Slowed riserdeployment and retrieval reduces availability and safety of the drillingrig, with important economic consequences. Fairings 214 suffer anotherdisadvantage, in that fairing sections are bulky, expensive, and subjectto damage when being deployed through the ocean surface wave zone.

FIGS. 3A and 3B show a semi-submersible drilling vessel 300. Suchvessels 300 are often configured as a platform 302 supported well abovethe ocean surface 304 on submerged longitudinal cylindrical buoyancypontoons 308. When beset by ocean currents 310, the pontoons 308, beinggenerally bluff, cylindrical objects, exhibit substantial hydrodynamicdrag due to flow separation. In order to maintain position relative tothe ocean floor 312, such vessels 300 are fitted with a system ofmoorings 314 and/or powered thrusters 316 to counter the drag forces.

Both moorings 314 and thrusters 316, however, are expensive, andmoorings 314 become impractical in very deep water. The presence ofmooring winches 318 adds substantially to the topside weight carried bysemi-submersible drilling vessels 300. This increase in weight reducesthe payload capacity of the vessel 300 and impairs its hydrostaticstability.

Hydrodynamic drag of a semi-submersible drilling vessel 300 can bereduced to a degree by rotating the vessel 300 so that the submergedpontoons 308 are aligned with the direction of current 310 and thoselocated down-current are relatively "shadowed" by those locatedup-current, as shown in FIG. 3B, which is an end-view of FIG. 3A. Newerdesigns of semi-submersible drilling vessels are intended to be moreazimuthally uniform in their hydrodynamic drag characteristics. Thisuniformity obviates directional drag reduction.

Boundary-layer-control ("BLC") has been investigated to attain high-lifton aircraft wings and to promote laminar, low friction, flow on wingsand elongated bodies. But no such system has been proposed or applied tofluid-submersed bluff bodies to reduce pressure drag and VIV. Unlikeaircraft wings, fluid-submersed hulls, such as SPARs, marine columns andrisers, and semi-submersible drilling vessels, are generally large,bluff, unstreamlined vertical circular-cylindrical forms or non-circularand/or horizontal cylindrical forms.

The presence of high drag levels and VIV on SPARs, marine risers, andsemisubmersible drilling vessels. Drag and VIV may prevent operation ofsuch ocean-deployed vessels, at a high cost to drilling operations.Thus, the inability to substantially reduce drag and VIV may have higheconomic costs.

Accordingly, while various systems and methods exist for reducing VIVand hydrodynamic drag in fluid-submersible objects, no such system ormethod reduces both VIV and hydrodynamic drag to a substantial degree.Moreover, while BLC has been applied to streamlined aircraft wings toattain high-lift, BLC has not been designed or applied to fluidsubmersedhulls, such as SPARs, marine risers and columns, and semi-submersibledrilling vessels. Accordingly, a need exists for a system and method forreducing VIV and hydrodynamic drag in fluid-submersed hulls and forthereby preventing suspension of drilling operations and other functionsin the presence of currents in the fluid.

SUMMARY

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

The present invention is a system for reducing the hydrodynamic drag andVIV on a bluff hull submersed at least partially in a fluid, such aswater. The term "bluff hull," as used herein, means vertical andhorizontal circular-cylindrical forms, vertical and horizontalelliptical-cylindrical and elliptical-semi-cylindrical forms, sphericalforms, semi-rectangular forms with rounded comers, and other bodies thatpresent a broad underwater profile against currents flowing in thewater. The term "bluff hull" thus includes SPARs, marine risers, andpontoons and floatation bodies for semi-submersible drilling structures,all of which are generally deployed in the ocean, in which swiftcurrents often travel that induce VIV and drag on a submersed bluffhull. The term "bluff hull," as used herein, also includes other typesof bluff bodies that are at least partially submersed in fluid,including bridge and pier stanchions and footings, ship and boat hulls,submarines, underwater communication cables, and underwater tunnelexteriors, to cite some examples. This definition of the term "bluffhull" also makes clear that the invention can be applied not only tofloating bodies, such as ships and SPARs, but also to bodies thatanchored to the bottom of the fluid, such as bridge stanchions andmarine risers. Also, the invention is useful in any fluid thatexperiences or conveys currents and other disturbances, but isespecially useful in oceans, seas, lakes, and rivers, all of which mayexperience strong and swift currents.

In a first embodiment, the present invention is a system for reducinghydrodynamic drag and VIV of a bluff hull beset by a current. The systemincludes a bluff hull, designed to be at least partially submerged in anexterior fluid. The bluff hull has a hull interior and a hull surface.The hull surface has an up-current side and a down-current side, suchthat the up-current side may be beset by a current present in theexterior fluid. The system also includes at least one opening of thehull surface for passing an interior fluid out of the hull interior intothe exterior fluid so as to reduce flow separation of the current on thedowncurrent side of the hull surface.

In another embodiment, the invention is a system for reducinghydrodynamic drag and VIV in a bluff hull. The system includes a bluffhull, designed to be at least partially submerged in water. The bluffhull has a cavity for holding water and a hull surface. The hull surfacehas an up-current side and a down-current side, such that the up-currentside may be beset by a current present in the water in which the bluffhull is at least partially submerged. The system also includes at leastone nozzle in the hull surface for blowing water from the cavity out ofthe hull surface and into the exterior fluid at a velocity greater thanthe current velocity and at an angle substantially tangential to thehull surface at the location of the nozzle and substantially in thedirection of the current, so as to reduce flow separation of the currenton the down-current side of the hull surface.

DESCRIPTION OF DRAWINGS

FIG. 1A is a side-view of a SPAR with prior art strakes.

FIG. 1B is a top-view of the SPAR of FIG. 1B along line B--B.

FIG. 2A is a side-view of a marine riser with prior art fairings.

FIG. 2B is a top-view of the marine riser of FIG. 2A along line B--B.

FIG. 3A is a side-view of a semi-submersible drilling vessel.

FIG. 3B is an end-view of the semi-submersible drilling vessel of FIG.3A.

FIG. 4A is a side-view of an exemplary embodiment of the system of thepresent invention.

FIG. 4B is a top-view of the embodiment of FIG. 4A along line B--B.

FIGS. 4C and 4D are top views of another exemplary embodiment of thesystem of the present invention showing a four slot-nozzleconfiguration.

FIG. 5A is a quarter-plan cut-away view at the pumproom level of a hullin accordance with the present invention.

FIG. 5B is a cut-away elevation-view along lines A--A of FIG. 5A.

FIG. 5C is a cross-sectional view of the hull of FIG. 5A parallel to thewater plane.

FIG. 6A is a perspective-end-view of an upper marine riser joint inaccordance with the present invention.

FIG. 6B is a cut-away perspective-end-view of a lower marine riserjoint, in accordance with the present invention, that can be coupled tothe upper marine riser joint of FIG. 6B.

FIG. 7A is a side-view of an exemplary embodiment that allows selectiveopening and closing of slot-nozzles, showing the slot-nozzle in a closedposition.

FIG. 7B is a side-view of the slot-nozzle of FIG. 7A in an openposition.

FIGS. 8A-8H show another embodiment that allows selective opening andclosing slots-nozzles.

FIG. 9A is a side-view of a marine riser in accordance with the presentinvention.

FIG. 9B is a cut-away view of the marine riser of FIG. 9A along lineB--B.

FIG. 9C is an enlarged-partial-view of the cut-away view of FIG. 9Bshowing a slot-nozzle and valve.

FIG. 10 is a plot of the distribution over depth of the velocityassociated with a once-in-10 years occurrence loop current in the Gulfof Mexico.

FIG. 11 is a plot of distribution over depth of the square of thevelocity, normalized to the maximum value found at the surface.

FIG. 12 is a plot illustrating a benefit of reducing the drag in theupper 200 feet of submergence of a marine riser.

FIG. 13 is a plot showing data on the drag coefficient (C_(d)) andStrouhal number (S) of vortex shedding for circular cylinders at highdiameter-Reynolds numbers (R).

FIG. 14 is a plot showing the distribution of time-averaged pressure(C_(p)) versus angle (θ) about a circular cylinder according to threemeasurements at relatively high diameter-Reynolds numbers, as well asthe theoretical pressure distribution for potential flow, i.e., ofinfinite Reynolds number.

FIG. 15 is a plot showing preliminary estimates of BLC characteristicsas applied to a marine riser.

FIG. 16 is a cross-section of a BLC-equipped body with a rectangularcross-section section and rounded corners, such as a horizontal pontoonused on a semi-submerged drilling vessel.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The present invention employs Boundary-Layer Control ("BLC") to reducehydrodynamic drag and VIV oscillations in fluid-submersed objects, suchas SPAR hulls, marine risers and columns, and sections ofsemi-submersible drilling vessels, that may be beset by ocean, river, orother currents.

As used herein, the term "fluid-submersible object" should not beconsidered limited to objects that are fully submersible in fluid (suchas the ocean), but rather broad enough to encompass any object that isat least partially submersed in fluid. For example, a marine riser, likethe riser 200 shown in FIG. 2, is completely submersed in the ocean,while SPAR 100 is only partially submersed. Moreover, as described inthe Summary above, as used herein, the term "hull" should be consideredto encompass many different forms and types of fluid-submersibleobjects, including vertical and horizontal circular-cylindrical forms,vertical and horizontal elliptical-cylindrical andelliptical-semi-cylindrical forms, spherical forms, and semi-rectangularforms with rounded comers, including SPARs, marine risers, pontoons andfloatation bodies for semi-submersible drilling structures, bridge andpier footings and stanchions, submarines, and ship and boat hulls.

Though this invention may apply to hulls at least partially submersed inany fluid, the remainder of this description will refer, forconvenience, to "water" as the fluid. This reference is not to beconsidered limiting on the invention.

FIGS. 4A and 4B are side-view and top-view, respectively, of anexemplary embodiment of the system of the present invention, showing avertically aligned, circular-cylindrical hull 400 equipped with BLC ofthe present invention. The exemplary inventive system of FIG. 4 includesblowing slot-nozzles 402 disposed on the surface of thecircular-cylindrical hull 400 symmetrically about a diameter alignedwith the onset current flow direction 404. For convenience, the term"slot-nozzle" will be used for the remainder of the description, meaningthat the nozzle is an elongated slot along the surface of a hull. Itwill be recognized, however, that any type of opening or outlet in thesurface of a hull that introduces high energy fluid in the direction ofthe flow of current and adjacent to the hull surface can be used toaccomplish BLC in accordance with the invention. For example, theopenings may be multiple point-nozzles, linear or slot nozzles,water-jet outlets, or any other opening through which fluid may beexpelled from the hull surface into the surrounding fluid. Preferably,however, the opening is a slot nozzle, which accelerates the velocity ofthe exiting fluid.

As shown in FIG. 1A, absent BLC the water in the boundary layer isretarded by friction and thus lacks the kinetic energy characteristic ofthe flow external to the boundary layer at the same angular position andis therefore subject to back-flow and separation in the face ofinevitable adverse pressure gradients, such as gradient 114. In theexemplary BLC system of FIG. 4A, the blowing slot nozzles 402 effecthigh-speed injection of water into the retarded boundary layer of theexternal current flow about cylindrical hull 400. The nozzles 402 blowmore energetic water flow 406 into the boundary-layer in a current-wiseA direction generally tangential to the cylindrical surface of the hull400 at an angular location generally up-current of the location ofuntreated boundary layer separation. To feed the nozzles 402, watercollected at a remote location or locations is passed through low-head,high-volume pumps to provide the higher energy water flow to the blowinglocations. Alternatively, the energization of boundary layer flow byblowing may be augmented at other up-current locations by suction into ahull of retarded boundary layer flow, which suction may provide part orall of the supply of water for blowing.

Together (or separately) the boundary layer blowing and suction actionsprevent separation of the current flow from the surface of thecylindrical hull (like the separation shown in FIG. 1B) at mostlocations downstream or aft of the diameter perpendicular to thedirection of onset current. This causes the external current flow to bedisposed more nearly like an ideal frictionless flow in its generalpressure distribution properties, as shown in FIG. 4B; with associatedsmall form drag. Absent separated flow, alternating vortex shedding,with its consequent induced lateral oscillation of the hull (or VIV, isalso minimized.

The BLC system can be used not only to reduce drag and VIV, but also topropel the hull 400 through the surrounding water, i.e., to dynamicallyposition the hull 400. The nozzles 402 can be selectively activated anddeactivated so as to blow water in a direction substantially opposite tothe desired direction of travel. This allows elimination of towingvessels, and on-board motors and screws, which are traditionally used tomove such hulls. Moreover, the BLC system can be used to correct hulltilt by high-low opposed blowing on the hull 400.

As described above and shown in FIG. 4, boundary layer energization isaccomplished through blowing nozzle-slots 402 imbedded in attached tothe surface 412 of hull 400. Preferably, and as shown in FIGS. 4A, 5A,and 5B, the nozzle-slots 402 are arranged vertically, coincident withgeometrical generator lines of the cylindrical hull surface 412, andgenerally perpendicular to the local components of the onset currentvelocities 404. Water is passed through, preferably, low-head,high-volume pumps 504 to the slots 402. The slots 402 are preferablyflush with the hull surface 412 and open immediately and continuouslyinto adjacent parallel conduits 506. The conduits 506 havecross-sectional areas much larger than that of the blowing slots 402,per unit slot length, and serve to distribute the blowing-water flowfrom a supply manifold 508 conveniently located along the verticallength of the slots 402.

The manifold 508 may be supplied by a pumping system that takes suctionfrom the sea in the "moon pool" 510 of the hull 400, or from anotherconvenient location. The pressurized water flow is accelerated throughthe slot-nozzles 402 to discharge externally in a direction tangentialto the local hull surface 412 and in the same direction as that of thelocal external current flow, as shown by reference numerals 406 and 404in FIGS. 4A and 4B. The blowing water exits the slot-nozzles 402 atgenerally the pressure of the local external flow at the nozzlelocation, but with a velocity adequately greater than that of the localexternal flow so as to energize the retarded boundary layer flow andthereby prevent flow separation. Preferably, the blowing slot-nozzles402 lie within the external contour of the hull surface 412 to avoidbeing damaged by the impact of water-borne objects. Blowing water may befiltered at sea suction inlet plenums 512 to exclude waterborneparticles of a size greater than the minimum blowing-slot opening.

In angular progression around the hull 400, clockwise- 514 andcounter-clockwise-516 directed slot-nozzles 402 are preferablyalternated, with a total population of 24 in intervals of 15 degreesbeing the preferred configuration. An adjacent pair of like-handedblowing slot-nozzles 402 will therefore be separated by approximately 30degrees in polar coordinates (as shown by reference numeral 518),centered on the center line 520 of the cylindrical hull 400, as seen inany plane cross-section of the hull 400 that is perpendicular to thecenter line. If dictated by the principles of flow physics, such blowingslots 402 may instead be separated by approximately 1/12 of thecircumference of the hull surface 412. Two such blowing slots, thoselocated farthest up-current, will preferably each be placed atapproximately 15 degrees (or 1/24 of the circumference of the hullsurface 412) down-current of the opposite extremes of the diameter thatlies nominally perpendicular to the onset water current direction. Asdictated by flow physics and economics, combinations of blowing slots402 may be replicated at locations yet further down-current, even withdifferent spacings, in order to further prevent flow separation from thehull 400 and the accompanying hydrodynamic drag and VIV-oscillations.

On a cylindrical hull or pontoon with a horizontal axis, the (elevation)angle of current incidence is approximately fixed at zero. In that caseas few as four blowing slot-nozzles may suffice, as shown in FIGS. 4Cand 4D.

In an alternative embodiment, blowing slots may precede suction slots(such as inlet plenum 512) in the flow-wise direction, or severalsuction slots may be grouped together at appropriate spacings and bepreceded or followed by several blowing slots similarly groupedtogether. It will also be appreciated that, as dictated by flow physicsand/or economy, BLC on a cylindrical hull may be accomplished by suctionslots only, with blowing discharge employed for propulsion thrustthrough discrete or continuous nozzles, or by slot-blowing means alonewith supply flow being supplied independently, or by any usefulcombination of such means.

The vertically lengthwise extent of any blowing or suction slot andassociated conduits, pairs of such slots and conduits, or sets of suchslots and conduits, may cover the entire length or height, or anyfraction of the length or height, of the hull, or some multiple of hulldiameter, as dictated by flow physics and/or economics. Therefore, BLCsystems may be contiguously replicated over the submerged verticalheight of a hull, from the water line to the hull bottom. These systemsmay be independent or interconnected as dictated by considerations ofcontrol, safety, economics, and redundancy requirements.

A "BLC system" always includes an opening in the hull surface throughwhich fluid is expelled into the surrounding water. A BLC systempreferably includes a plurality of slot nozzles discharging fluid into acurrent stream of fluid outside the hull surface in a directionsubstantially tangential to the hull surface and substantially parallelto the outside current stream. This discharged fluid energizes aboundary layer in the fluid immediately adjacent the hull surface inorder to prevent subsequent separation of the current stream from thehull surface. The BLC system may also include a plurality of contiguousconduits internal to the hull surface that supply by pressurized fluidto the nozzles and that are supplied with pressurized fluid from pumpsor other convenient pressurized fluid source via appropriate piping anddistributing manifolds. The BLC system may also include filters thatfilter the fluid supply to the pumps drawn from the body of fluidexternal to the hull. The fluid supply may either be directly directlysupplied to the pumps or may be supplied through boundary layer suctioninlets on the hull surface. The BLC system may also include acomputer-control system that is coupled to a sensing system located onthe outer surface of the hull for the purpose of determining thedirection and strength of the onsetting current stream andelectronically controlling the selection and opening or closing of thenozzles required to energize the boundary layers and avoid flowseparation in the most economical and effective manner.

The hull 400 may be rotatably-configured so that it always presents thesame aspect to the onset water current 404, without regard for thegeographic direction of the current 404. For example, the hull 400 shownin FIG. 4 has two sets of vertically-aligned slots 402 located onopposite sides of the circular-cylindrical hull 400. Assuming the hull400 can be rotated about its vertical axis 408, if the direction of thecurrent 404 changes, the hull 400 can thus be moved so that the two setsof slots 402 are always aligned at approximately a 90 degree angle 410relative to the onset current 404. Moreover, the slots 402 would eachdischarge in a down-current direction generally tangentially to the hullsurface 412 in which they may be imbedded. A rotatable hull 400 permitsfewer sets of vertically-aligned slots 402, because the hull 400 canalways be rotated to optimally align the slots 402 to the current 404.If, however, hull rotations are to be limited to ninety degrees ineither direction, then two pair of blowing slot-nozzles 402 should beprovided, at a minimum. The two pairs will be arrayed in selectablyclockwise and anti-clockwise blowing pairs located at the extremes ofthe perpendicular diameter.

Alternatively, if the hull 400 cannot be rotated about its vertical axisin order to always present the same aspect to the onset water current404, then the configuration of the BLC system will be selected to alignit for best drag and VIV reduction. One method for accomplishing thisgoal is to fit the hull with an adequate number of vertically-alignedblowing and/or suction slots (assuming the hull is vertically oriented,such as hull 400), with the slots spaced equally around the periphery ofthe hull to allow BLC configuration by selective activation ofappropriately located blowing and/or suction units. An example of thisconfiguration is shown in FIGS. 5A AND 5B, in which the slots 402 arespaced around the hull 400 in equally-spaced increments of 15 degrees.Note that, in FIGS. 5A AND 5B, the blowing slots 402 are doublyrepresented because water blowing in either clockwise 514 oranti-clockwise 516 tangential directions may be required at anyparticular angular location.

The blowing and/or suction manifolds 508 will preferably take the formof rings, as shown in FIG. 5C, which is a cross-sectional view of thehull 400 parallel to the water plane. Such a ring blowing manifold 508will communicate with each proximate blowing conduit 506 through aremotely operated valve 522. Similarly, such a ring suction manifold 508will communicate with each proximate suction conduit (not shown) througha remotely operated valve, like valve 522. A computer controlledalgorithm can be used to select the set of blowing and/or suction slotsthat will most symmetrically accommodate the onset current flow. Thealgorithm can also be designed to set the BLC system, as assembled, intooperation to most economically and effectively reduce hydrodynamic dragand VIV.

In the case of a marine riser structure, such as that shown in FIGS. 2AAND 2B, it is preferred that the selector valves 522 be integral withthe blowing-slot-nozzles 402, which will preferably be remotelycontrollable. This arrangement allows all of the conduits 506 to be usedto transport blowing-water from the top of the riser stack downward.

Over the draft of a SPAR vessel, such as that shown in FIGS. lA and lB,the vertical length of the SPAR 100 may exceed 600 feet, potentiallyresulting in considerable variation in the velocity of an onset currentalong the SPAR's vertical length. Maximum current velocities occur atthe surface and continue strong to depths of approximately 200 feet,beyond which the current velocity is found to decrease sharply and thento attenuate more slowly in greater depths. As the required velocity andvolume of BLC water-blowing is markedly dependent upon the velocity ofthe onset current, economy may dictate that the water-blowing system besegregated into two parts: one at a typically higher blowing-slotvelocity and volume-per-unit length, serving shallower areas of thesubmerged hull, and another at a typically lower blowing-slot velocityand volume, serving the deeper areas of the hull. For example, assuminghull 400 were a several hundred foot long SPAR, the top two sets ofblowing slots 402 could be set to expel water at a higher velocity thanthe three lower sets of blowing slots 402. Such a configuration wouldrequire a replication of blowing-water ring manifolds 508, one for thetwo higher sets of slots and their higher blowing-water pressure, andanother for three lower sets and their lower blowing-water pressure.

If multiple velocity blowing slots are employed, some of the pumps 504will discharge to the high head manifold and be designed and operated toprovide such head and flow as appropriate, while the remainder of thepumps 504 will discharge to the lower head manifold for lower velocityflow. Each blowing-slot 402 will be separated into independent upper andlower parts at approximately 200 feet depth location, and each such partwill be supplied by a separate vertically running conduit 506. Thetwo-fold set of conduits 506 will be controllably connected through atwo-fold set of remotely operated valves 522 to the high-and low-headsupply manifolds. Pump suction, however, may remain common. Allvertically running conduits 506 may be formed between radial-verticalpartitions extending between concentric inner hull skin 524 and outerhull skin 412.

Alternatively, and preferably with a BLC-equipped marine riser, onepumping system and manifold would be retained, but the vertical supplyconduits may be fitted with chokes at appropriate depths to apportionand limit the blowing-water flow proceeding downward. Slot-nozzle throatareas may also be reduced with increasing depth position.

A computerized control system (not shown), coupled to electro-mechanicaldevices, such as valves 522 and pumps 504, may be used to control theBLC system. Input signals to the control system relaying currentstrength and relative direction may be generated manually, by remotesensors (not shown), or by an array of pressure transducers around thecircumference of the hull 500. In a preferred automated control system,the depth of BLC-equipped hull would be divided into zones, each ofwhich would be monitored by a ring array of hull surface pressuretransducers and controlled by a submerged battery powered generalpurpose or custom, dedicated computer. A pattern recognition algorithmapplied to the pressure distribution would provide an estimate ofrelative current direction and magnitude, in known fashion. Eachcomputer would translate such information to select the particularblowing slot-nozzle pairs to activate in their zones and collectively todemand pump speed.

FIG. 6A shows the end of an upper marine riser joint 600 equipped with aBLC system in accordance with the present invention. A plurality of stub"pin" tubes 602 pierces a lower coupling flange 604 of the upper riserjoint 600 and preferably protrudes below the flange 604 approximately 8inches. The stub tubes 602 bear o-rings in suitable grooves for makingthe subsequent pipe connection pressure-tight, in known fashion. Asshown in FIG. 6B, which is a cut-away view of a lower riser joint 606that can be mated to the upper riser joint 600, a mating upper couplingflange 608 is pierced by a plurality of "box" tubes 610 whose innerdiameter is slightly larger than the outer diameter of the mating pintubes 602. When "stabbed" together, the box tubes 610 and pin tubes 602form pressure-tight connections between corresponding BLC conduits 612of the upper riser joint 600 and lower riser joint 606.

A plurality of such pin/box tube connections may be arranged in a ringwhose radius is approximately equal to the mean radius of the BLCconduits 612. These pin/box tube connections are similar in mostrespects to those of a choke line 916 and a kill line 918, both of whichalso pass through the riser coupling flanges, but at a smaller radiusthan the pin/box tube connections. The choke line 916 and kill line 918will be described below in connection with FIG. 9B.

A pin tube 602 of the upper riser joint 600 extends above the lowerflange 604 a short distance to where it is permanently (or removably)joined to and through a lower end plate (not shown) of a BLC conduit612. Similarly, a box tube 610 of the lower riser joint 606 extendsbelow the upper flange 608 a short distance to where it is permanently(or removably) joined to and through the upper end plate 614 of a BLCconduit 612. When the riser coupling flanges 604, 608 are connectedtogether, the stabbed-together pin tubes 602 and box tubes 610 eachhydraulically connect a single BLC conduit 612 of an upper riser joint600 to a single BLC conduit 612 of a lower riser joint 606. A pluralityof such connections forms separate, parallel connections between all ofthe BLC conduits 612 of an upper riser joint 600 and all of the BLCconduits 612 of a mating lower riser joint 606.

The mating coupling flanges 604, 608 are held and forced together toform a preferably rigid connection between adjacent riser joints 600,606, preferably by an external clamp (not shown). The clamp is locatedexterior to the flanges 604, 608 and to all BLC conduits 612, buoyancymaterial, and wiring that may be surrounding the riser pipe.

At a vertical location a short distance above the lower end plates 614of the BLC conduits 612 of the lower riser joint 606, radial-verticalpartitions 616 that separate adjacent conduits 612 are preferablylaterally pierced 618 to allow and ensure equalization of the pressureand blowing-water flow quantity conducted by each of the connected boxtubes 610, by means of cross-flow among them. Similarly, at a verticallocation a short distance below the upper end plates 614 of the BLCconduits 612 of the lower riser joint 606, the partitions 616 arelaterally pierced 618 to allow and ensure equalization of the pressuresand flow quantities conducted among the conduits 612 below.

At selected locations of riser joint connections, one or several of theconnecting tubes 602, 610 may be blanked or choked in order to reducethe flow rate proceeding below. This may be done to match, more closelyand more economically, the resulting jet velocity and flow rate to thegenerally decreasing current velocity found with increasing depthlocation.

Preferably, slot-nozzles 620 are provided that are of a length slightlyless than the length between flanges of a riser pipe joint. Theseslot-nozzles 620, when opened, communicate between their associatedconduits 612 and the sea surrounding the riser 606. In cross section,when opened, these slot-nozzles 620 have the general configuration ofconverging nozzles that discharge the blowing-water at high velocity ina direction generally tangential to the external, circular-cylindricalsurface 622 of the riser 606.

Adjacent slot-nozzles 620 are configured to discharge alternately in theclockwise 624 and counter-clockwise 626 directions, sequentially.Preferably, the spacing of like-direction discharging slot-nozzles 620is nominally 30 degrees in angular position. The angular spacing ofslot-nozzles 620 is thus, preferably, nominally 15 degrees. Inconsequence, the preferable configuration has 24 slot-nozzles 620equally spaced about the external circumference of the riser structure606, including 12 slot-nozzles 620 discharging in a tangentiallyclockwise direction 624, uniformly and alternatingly interspersed with12 slot-nozzles 620 discharging in a tangentially counter-clockwisedirection 626. All such slot-nozzles 620 extend between the terminatingend plates 616 of their proximate, associated water conduits 612. In thepreferred embodiment, the slot-nozzles 620 are generally closed,preventing water discharge until deliberately and selectively opened.

An embodiment that permits selective opening of slot-nozzles 620 isshown in FIGS. 7A and 7B. In this embodiment, slot-nozzles 620 haveadjacent, substantially parallel, elongated compartments 702 that arekidney-shaped in cross-section. These parallel compartments 702 arenormally vented to the sea through one or more small holes 704 at theends of the compartments 702. When, by means of a remotely operatedvalve 706, the substantially parallel compartments 702 are opened to thepressurized water in the adjacent blowing-water conduit 612, thecross-section of the compartments 702 becomes more rounded, as shown byreference numeral 708. This distortion of the adjacent parallelcompartment 702 causes the slot 620 to open, as shown in FIG. 7B,releasing a high-velocity wall-jet of water into the sea in a directionsubstantially tangential to the hull surface 622. Therefore, byselection of the valve 706 to open between selected parallel compartment702 and blowing-water conduit 612, and by opening the valve 706, theimmediately adjacent slot-nozzle 620 is opened, and the selectedblowing-water jet is activated. While the control valve 706 is active, asmall flow of high pressure water leaks from the vent holes 704, fromparallel compartment 702 to sea. Preferably, the leakage flow issupplied by the control valve 706 at a pressure nominally equal to thatof the water in the conduit 612.

Conversely, when a selected open control valve 706 is closed by remoteaction, the small flow of high pressure water is terminated, and, byleakage through the bleed hole 704, the pressure in the parallel controlcompartment 702 is promptly reduced to sea pressure. With this pressureequalization, the shape of the control compartment 702 cross-sectionreturns to its kidney-shaped form of FIG. 7A because of the elasticityof the plastic material of which it is preferably formed. This return tothe kidney shape causes the adjacent slot-nozzle 620 to close, as shownin FIG. 7A.

Alternatively, a normally open slot-nozzle 620 may be kept closed by thepressurized inflation of an adjacent parallel control compartment 702 ofsuitably shaped cross-section. The selective opening of a slot-nozzle620 so configured is caused by the closure of its associated controlvalve 706, which cuts off the inflating flow from, and communicationwith, the high pressure water in the proximate supply conduit 612.Preferably, however, the slot-nozzle 620 is kept normally closed in theabsence of selected control valve 706 opening, and, due to theelasticity of the compartment 702, the control valve 706 resists theinternal pressure of the water in the supply conduit 612.

FIGS. 8A-8H show an alternative configuration of a slot-nozzle 620 withassociated valve. A linear valve 802 separates the slot-nozzle 620 fromthe proximate blowing-water conduit 612, the valve 802 being formed by aperforated plate-strip 804 that is slideably connected to a matchingmultiplicity of ports 806 in the stationary body 808 of the valve 802.The shape of the port-perforations 812 in the valve slide 804 matchesthat of the ports 806 in the valve body 808 except for a small reductionin the lengthwise aperture extent of the prot perforations 812. When thevalve slide 804 is positioned to align the port apertures 812 with thoseof the valve body 808, as shown in FIGS. 8A-8D, the adjacent selectedslot-nozzle 620 is opened against its elastic constraint by the pressureof the then-communicated blowing-water, and the slot-nozzle 620 isactivated. When the valve slide 804 is moved longitudinally byapproximately one-half the pitch of the port spacing, the flow ofblowing-water is cut off, the pressure of the water in the ante-chamber808 of the slot-nozzle 620 drops promptly to the sea pressure, and theslot nozzle 620 closes by reason of its elasticity, as shown in FIGS.8E-8H.

The lengthwise positioning of the valve slide 804 may be effected by awater-powered movable piston in a cylinder (not shown) that makes use ofthe pressure difference between the blowing-water in the supply conduit612 and that of the surrounding sea. The pressurized water may beadmitted to the actuating cylinder by a remotely actuated valve in themanner described above, and, when the valve is subsequently closed, itwill allow reverse motion of the piston and connected valve slide by aspring while cylinder water is vented to the sea via a small bleed hole.

FIGS. 9A-9C show other features of a marine riser 900. At or near thewaterline 902, below the sliding joint (not shown) at the top of theriser stack, a ring manifold 508 may be clamped to a flanged connectionbetween two riser joints. Alternatively, and preferably, such a ringmanifold 508 will form the lower part of a special riser pup-joint. Thisring manifold 508 on its lower side exhibits a plurality of stub pinpipes 604 that are stabbed into a matching plurality of box tubes 610that, in turn, penetrate the upper end plates of the BLC conduits 612 ofthe uppermost riser joint to which the BLC system is applied. All of theblowing-water supplied to the BLC system for the entire riser passesthrough this ringmanifold 508. One or more flexible water feed pipes orhoses 904 are connected to and through the upper side of the ringmanifold 508. The feed pipes or hoses 904 extend upward to a deck of thedrilling platform (not shown) where they are distally connected to thepump or pumps that supply the pressurized slot-blowing water serving theentire BLC system of the riser 900.

As can be seen in FIG. 9B, which is a cut-away along line B--B of FIG.9A, the riser 900 includes several blowing-water conduits 612 arrangedperipherally about the outside of the riser 900. FIG. 9B also shows apair of blowing slot-nozzles 620, arranged to blow water substantiallytangentially to the riser's surface 622 and in the onset direction ofcurrent 906. The riser 900 also includes buoyancy material 908, arrangedin a ring centered about the longitudinal axis of the riser 900, with ariser pipe 910 being arranged in a ring inside the buoyancy material908. In addition, a ring-shaped passage 912 to carry drilling mud isarranged in a ring shape within the the riser pipe 910, and a drillstring 914 is located at the center of the riser 900. As noted above,the riser 900 may also include the choke line 916 and the kill line 918,in known fashion.

FIG. 9C is a blow-up of FIG. 9B, showing a cross-section of aslot-nozzle 620 with a linear valve 920 that controls the flow of waterto the slot-nozzle 620. The slot-nozzle nozzel 620 opens to the sea todeliver high-velocity water in a direction substantially tangential tothe hull surface 622 when the valve 920 is open to the blowing-waterconduit 612.

FIG. 10 is a plot of the distribution over depth of the velocityassociated with a once-in-10 years-occurrence loop current in the Gulfof Mexico. The maximum current occurs at the surface of the sea andexhibits about 3.5 kts. velocity. This extreme current and its variationwith depth defines the environmental threat to the offshore structuresthat the drag and VIV-reducing BLC system of the present invention isdesigned to overcome. Significant is the bimodal nature of the currentvelocity distribution where the greatest velocities are found above 200feet depth.

FIG. 11 is a plot of distribution over depth of the square of thevelocity associated with the Gulf of Mexico's once-in-10years-occurrence loop current, normalized to the maximum value found atthe surface. Assuming the drag coefficent of a circularly-cylindricalhull such as a SPAR or a marine riser is independent of its position indepth, this plot reflects the relative strength of the locally actingdrag force. This plot further illustrates the concentration of dragforce in the upper 200 feet of submergence depth. It suggests that dragreduction in this region would be most productive and cost effective.

The plot in FIG. 11 contains averages over 1000 feet of depth of thevelocity, its square and its cube, each normalized by the correspondingfunction of the velocity at the surface, 3.5 kts. The linear averageshows that, if the local blowing-water flow rate is made proportional tothe local onset current velocity, the total blowing-water flow ratewould be only 57% of that which would obtain if the blowing-water flowrate were everywhere matched to the surface current velocity. Thedepth-averaged current velocity is only 2.00 kts. The square root of thedepth-averaged square of the local velocity is only 2.13 kts, and thetotal drag, being proportional to the square of the velocity (diameterand drag coefficient presumed constant) is only 37% of that if thesurface current were to be maintained over the 1000 foot depth extent.Similarly, the power required to pump the blowing-water, without loss,if local jet velocity were a constant multiple of local currentvelocity, would be only 27% of that which would obtain if the surfacecurrent velocity value were maintained over the depth extent. It isclear, therefore, that an economy will be obtained by attempting toapportion, at least roughly, the local blowing jet velocity to the localonset current velocity. This conclusion applies to both marine riser andSPAR vessel applications. For floating drilling vessels, however, thedraft is generally entirely within the 200 foot depth range of thegreatest current velocities. Thus, current velocity matching appearsunnecessary.

FIG. 12 illustrates a benefit of reducing the drag in the upper 200 feetof submergence of a marine riser. This figure is the result of ananalysis in which the following conditions were assumed: the riser isdeployed in deep water in the Gulf of Mexico (more than 4000 feet ofwater depth); the 54" riser structure is beset by the current velocityprofile of FIG. 10, resulting in an unmitigated drag distributionproportional to that of FIG. 11; the upper balljoint (or gimbal joint)is maintained vertically above the lower balljoint at the sea bed, inknown fashion; the riser structure is everywhere rendered neutrallybuoyant by the application of distributed buoyancy material, such asbuoyancy material 908 shown in FIG. 9; the bending stiffness of theriser structure is governed by the applied tension as though it were astring, rather than as a beam (which is indeed quite true for tensionvalues greater than one million pounds); the riser structure, because ofVIV, suffers a drag coefficient (C_(d)) of 1.5 in its unmitigated state;and BLC entirely removes the drag from the topmost 200 feet.

The upper curve of FIG. 12 (indicated by squares) shows the estimatedangle at the upper balljoint resulting from the applied drag forceswithout any drag reduction, as a functions of applied riser tension. Itis clear that increased tension stiffens the riser and reduces theballjoint angle induced by those drag forces. Balljoint angles greaterthan about two degrees may cause cessation of frilling because ofbinding of the drill string which runs through the riser, withconsequent severe economic consequences. Additionally, the balljointangle is indicative of bending strain, hence stress, in the riser, towhich is added that due to tension, and finally an alternating stressdue to VIV. The sum of these stresses threatens fatigue failure of theriser structure.

The lower curve of FIG. 12 (indicated with dots) shows the estimatedupper balljoint angle if the drag on the upper 200 feet of riser isremoved along with its associated VIV. This illustrates the benefits inoperability of the rig, as well as the reduction in associated riserstresses and fatigue that may be obtained by a successful, limitedapplication of BLC drag and VIV reduction.

FIG. 13, taken from A. Roshko, "Experiments on the Flow Past a CircularCylinder at Very High Reynolds Number, 10 Journal of Fluid Mechanics at345-56 (1961), shows data on the drag coefficient (C_(d)) and Strouhalnumber (S) of vortex shedding for circular cylinders at highdiameter-Reynolds numbers (R). The referenced data reported and plottedby Roshko is at the highest values of Reynolds numbers known to theinventors, i.e., approaching 10⁷. By comparison, the Reynolds number fora 54 inch diameter riser in a sea water current of 3.5 kts. isapproximately 2.7×10⁶, which is within the reported data range, whilethat for a 120 foot diameter SPAR vessel in the same current is about7.2×10⁷, which is beyond the reported data range.

It may be seen that the drag coefficient for the riser case isapproximately 0.8 and generally locally increasing steeply from lowervalues at somewhat smaller Reynolds numbers. It should be noted thatRoshko's drag data were acquired on a smooth cylinder model with adownstream splitter plate fitted to prevent vortex shedding, renderingthe drag and pressure distribution on the cylinder model to be timeinvarient. Drag coefficients reported by Delaney and Sorensen (perRoshko FIG. 13) are quite low. An assumption of a drag coefficient equalto 1.0 for a real, somewhat rough cylinder in a high interval ofReynolds number is not unreasonable, provided VIV is absent. With VIV,the drag coefficient may be doubled.

The Strouhal number (S) for vortex shedding (defined as the product ofshedding frequency in Hertz multiplied by diameter and divided bycurrent velocity), is seen to be a good deal smaller according toRosbko's data and than that of Delaney and Sorensen at slightly smallerReynolds numbers. Roshko also shows a general moderate increase inStrouhal number with increasing Reynolds number within the range of hisdata. The value of S deducible from Roshko's data appears to be about0.27, which is a good deal higher than the value of 0.2 that is nominaland apparently consistent with a drag coefficient of 1.0, according toRoshko's plots. Extrapolating Roshko's data, one would expect theshedding Strouhal number for a SPAR vessel to be about 0.3 or evenhigher. Thus, without BLC mitigation, the frequency of VIV motions, withconsequent accelerations and structural fatigue, are increased relativeto expectations for large structures beset by ocean currents at highReynolds numbers.

As is apparent from the above and by actual experience, the drag and VIVof large cylinders is not mitigated by virtue of their very highReynolds numbers. The conclusion is that drag and VIV reduction by BLCis desirable.

FIG. 14, also taken from Rosbko, is a plot showing the distribution oftime-averaged pressure (C_(p)) versus angle (θ) about a circularcylinder according to three measurements at relatively highdiameter-Reynolds numbers, as well as the theoretical pressuredistribution for potential flow, i.e., at infinite Reynolds number. Theangular coordinate of the abscissa is measured from the "nose" at zerodegrees to the "tail" at 180 degrees. The pressure coefficient of theordinate represents the difference between local pressure on thecylinder surface and that in the onset flow at a distance (at the sameheight) divide by the dynamic pressure of the onset flow velocity.

It is apparent from FIG. 14 that, between the angles of 30 and 150degrees relative to the onset current, the surface pressure in potential(ideal, frictionless) flow is lower than in the environment. Thepressure is lowest at 90 degrees to the current vector. However, sincethe pressure distribution on a circular cylinder in potential flow issymmetric about a diameter perpendicular to the onset flow (90 degrees),there is no net pressure drag in the ideal flow case, which paradoxicalresult is indeed predicted by theory.

The reported experimental data, however, show considerable asymmetry,thereby yielding substantial pressure drag. Over the forward part of thecylinder (0-90 degrees), the measured pressures tend to approach thepotential flow values increasingly with Reynolds number, although theeffects of boundary layer development are apparent. The pressures on theafter part of the cylinder (90-180 degrees), depart substantially fromthe potential flow values, reflecting separated flow, and in a seeminglyunsystematic manner relative to Reynolds number. Considering thefor-and-aft projected areas of the cylinder upon which these pressuresact, the associated pressure drag forces are seen to be consistent withthe drag coefficient variations with Reynolds number seen in FIG. 14.The ultimate objective of the BLC system is to return the cylinderpressure distribution to a more nearly symmetric form, and therebyreducing or eliminating pressure drag and associated VIV. In any event,pressure distributions such as these must be sensed and interpreted bythe BLC control system in order to estimate the direction and strengthof the onset current.

FIG. 15 is a plot showing preliminary estimates of BLC systemcharacteristics as applied to a marine riser. The surface value of the10 year loop current in the Gulf of Mexico (see FIG. 10) is assumed asapproximately 6 fps. The "parameter" value of 6 represents the ratio ofthe momentum inserted into the boundary layer at each blowingslot-nozzle to the momentum deficit of that boundary layer approachingthat location. An additional measure of conservatism is manifest in thefactor of 2 applied to the friction drag coefficient that determines theboundary layer's momentum deficit.

With the stated diameter of the riser cylinder, these parameter valuesyield a value of about 0.05 for the blowing coefficient (Cy) whichrepresents the ratio of single slot-nozzle thrust-to-cylinder drag atunity drag coefficient. Extrapolating the results of J. E. Hubbartt andL. H. Bangert, "Turbulent Boundary Layer Control by a Wall Jet," AIAAPaper 70-107, 8th Aerospace Sciences Meeting Jan. 19-21, 1970 (NewYork), which results were obtained at a lower Reynolds number thandescribed above, gives assurance that, with a momentum ratio of 6,complete avoidance of flow separation on a riser with its associatedpressure drag and VIV will be avoided.

FIG. 15 exhibits the blowing jet velocity (circle symbol), the ratio ofsame to local current flow velocity outside the boundary layer at theslot location (triangle symbol, point down), the flow rate (squaresymbol), and the blowing water horsepower (triangle symbol, point up);the latter two: per unit height of riser are all plotted against the gapheight of the blowing nozzle slots. A slot gap height of 0.014" has beenselected, which requires a jet velocity of 60 fps, ten times the onsetcurrent velocity at the sea surface and five times the local currentvelocity. The flow required is 1.1 g.p.s. per foot of riser height. Thepumping water horsepower is about 0.9 HP/ft, which compares favorablywith the noted unmitigated drag power of 1.74 HP/ft.

FIG. 16 is a cross-section of a horizontal pontoon 1600 of asemi-submerged drilling platform (not shown). As shown here, thehorizontal pontoons 1600 of a semi-submerged drilling platform, as wellas, occasionally, the vertical columns, generally have a rectangularcross-section with flat sections 1602 and rounded comers 1604. Toprovide BLC for the pontoon 1600, longitudinal blowing slot-nozzles 1606are fitted at the intersections of the arcs of corners 1604 and theadjacent flat sections 1602 that form the sides 1608, 1609, top 1610,and bottom 1612 of the pontoon 1600. There are therefore, preferably,eight such slot-nozzles 1606 arrayed along the length of the pontoon1600.

In a transverse section of the pontoon 1600, a pair of tangentiallydischargeable slot-nozzles 1606 are installed at each comer 1604. Eachsuch slot-nozzle 1606 discharges in a direction parallel to the localcomponent of current velocity, generally from the flat section 1602toward the arc of the rounded comer 1604. As in both the SPAR and marineriser applications, each such slot-nozzle 1606 is supplied blowing-waterby means of a proximate longitudinal conduit 1614 charged by a pumpingsystem (not shown).

Assuming FIG. 16 is looking forward, when beset by a current from a portside 1616, the pair of slot-nozzles 1606 bounding the port side panel1608 are selectively activated 1618, 1619 to assist the flow to traversethe comers 1604 facing the current 1616 without separation. Further, thepair of slot-nozzles 1606 rightwardly bounding the top 1610 and bottom1612 panels of the pontoon 1600 are selectively activated 1620, 1621 tore-energize the boundary layers exiting the top 1610 and bottom 1612surfaces and prevent flow separation with its associated pressure dragon the starboard side 1609 of the pontoon 1600. When the pontoon 1600 isbeset by a current from the starboard side, the mirror image of theabove arrangement obtains.

A number of embodiments of the present invention have been described.Neverthe-less, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

What is claimed is:
 1. A system for reducing hydrodynamic drag andvortex-induced-vibration ("VIV") of a bluff hull beset by a current,comprising:a. a bluff hull, designed to be at least partially submergedin an exterior fluid, the bluff hull having a hull interior and a hullsurface, the hull surface having an up-current side and a down-currentside such that the up-current side may be beset by a current present inthe exterior fluid, wherein the bluff hull has an axis and issubstantially cylindrical with the axis running along the height of thecylinder, and wherein the current has a current velocity, and b. atleast one opening of the hull surface for passing an interior fluid outof the hull interior into the exterior fluid so as to reduce flowseparation of the current on the down-current side of the hull surface,wherein the opening passes the interior fluid out of the hull interiorat an angle substantially tangential to the hull surface at the locationof the opening and substantially parallel to the direction of thecurrent, wherein the opening blows the interior fluid out of the hullinterior at a blowing velocity greater than the current velocity,wherein the opening is a slot-nozzle substantially aligned with the axisof the bluff hull; c. the at least one opening comprising a plurality ofslot-nozzles substantially aligned with the axis and disposed around thesurface of the bluff cylindrical hull; and d. a control system forcontrollably opening and closing the plurality of slot-nozzles, whereinthe control system includes a hydraulic system for opening and closingthe plurality of slot-nozzles and includes a valves and wherein thehydraulic system includes a valve, coupled to a bladder, forcontrollably passing high-pressure fluid into the bladder to expand thebladder, the bladder being coupled to a slot-nozzle such that when thebladder is expanded the slot-nozzle is open.
 2. A system for reducinghydrodynamic drag and vortex-induced-vibration ("VIV") of a bluff hullbeset by a current, comprising:a. a bluff hull, designed to be at leastpartially submerged in an exterior fluid, the bluff hull having a hullinterior and a hull surface, the hull surface having an up-current sideand a down-current side such that the up-current side may be beset by acurrent present in the exterior fluid; b. at least one opening of thehull surface for passing an interior fluid out of the hull interior intothe exterior fluid so as to reduce flow separation of the current on thedown-current side of the hull surface; c. a cavity in the hull interiorfor holding the interior fluid; and d. a conduit, located in the hullinterior and coupled to the cavity and to the opening, for conveying theinterior fluid to the opening.
 3. The system of claim 2 wherein theopening is a slot-nozzle having a vertical length and the conduit is anelongated space disposed along at least the vertical length of theslot-nozzle.
 4. The system of claim 3 wherein the bluff hull is a marineriser having a vertical length, having a plurality of slot-nozzles thatopen at the surface of the marine riser, and having a plurality ofconduits each corresponding to at least one of the slot-nozzles, theslot-nozzles and conduits being disposed along the vertical length ofthe marine riser.
 5. A system for reducing hydrodynamic drag andvortex-induced-vibration ("VIV") in a bluff hull, comprising:a. a bluffhull, designed to be at least partially submerged in water, the bluffhull having a cavity for holding fluid and a hull surface, the hullsurface having an up-current side and a down-current side such that theup-current side may be beset by a current present in the water in whichthe bluff hull is at least partially submerged, the current having avelocity, wherein the bluff hull has an axis along a long dimension ofthe bluff hull; b. at least one nozzle in the hull surface for blowingthe fluid contained in the cavity out of the hull surface and into thewater at a velocity greater than the current velocity and at an anglesubstantially tangential to the hull surface at the location of thenozzle and substantially in the direction of the current so as to reduceflow separation of the current on the down-current side of the hullsurface; and c. a plurality of slot-nozzles disposed at the hull surfacealong the lone dimension of the bluff hull, each slot-nozzle for blowingfluid held in an internal cavity of the bluff hull out of the hullsurface and into the water at an angle substantially tangential to thehull surface and substantially in the direction of the current; and d. acontrol system for selectively activating and deactivating eachslot-nozzle.